Some products of semiconductor devices, such as insulating gate bipolar transistors (IGBTs) and MOS field effect transistors (MOSFETs), include diodes for sensing temperature (hereinafter, referred to as temperature sensing diodes) formed on semiconductor chips.
FIG. 18 is a view illustrating a schematic configuration of a conventional temperature sensing diode 500. FIG. 18A is a plan view of the main portion, and FIG. 18B is a cross-sectional view illustrating a cross-sectional structure taken along a line III-Ill of FIG. 18A. FIG. 18A also illustrates a current path.
The conventional temperature sensing diode 500 is obtained by forming a silicon oxide film 57 on a silicon substrate 51 with semiconductor devices, such as MOSFETs, formed on the silicon substrate 51, and forming an n-type region (a cathode region) 64 and a p-type region (an anode region) 65 by impurity doping in a polysilicon layer 58 grown on the silicon oxide film 57. The temperature sensing diode 500 detects the temperature of the semiconductor chip by using the temperature characteristics of forward voltage drop Vf. A pn-junction interface 73 is located in the middle between the a first contact hole end 68a of a first contact hole 68 and a second contact hole end 69a of a second contact hole 69, the first contact hole 68 and the second contact hole 69 being formed in an interlayer insulating film 66. The pn-junction interface 73 is located at the center of an interlayer insulating film 66a sandwiched between the first contact hole 68 and the second contact hole 69.
Applying constant current I (current not larger than mA) to the temperature sensing diode 500 causes forward voltage drop Vf between the anode and cathode of the temperature sensing diode 500. The forward voltage drop Vf has the property of decreasing with an increase in temperature. The temperature sensing diode 500 is a device to detect temperature by using this property.
FIG. 19 is a diagram illustrating the relationship between the forward voltage drop Vf and temperature T in the conventional temperature sensing diode 500. When the temperature T increases from 25° C. to 150° C., Vf is decreased by about 20% to 30%. Variation of Vf causes variation of detected temperature Ts with respect to forward voltage drop Vfo which is set for detection.
Vf of the temperature sensing diode 500 is the sum of voltage Vpn generated at the pn-junction interface 73 by applying the current I and voltage (I×Rpn) generated across a parasitic resistance Rpn, which is the sum of parasitic resistances Rp and Rn of the n-type region 64 and the p-type region 65. Vpn depends on the internal potential at the pn-junction interface 73. Vf is expressed by:Vf=Vpn+I×Rpn 
Generally, ion implantation to form the n-type region 64 and the p-type region 65 in the temperature sensing diode 500 concurrently serves as an ion implantation process to form a semiconductor device. Accordingly, the ion implantation dose, implantation energy, activation heat treatment, and the like are restricted by the process conditions to form the semiconductor devices. It is therefore difficult to control the impurity profile of the temperature sensing diode 500 alone. At the ion implantation into the polysilicon layer 58, some of the implanted impurity ions penetrate the polysilicon layer 58 by the channeling phenomenon (the range of ion implantation into polysilicon is longer than the range of ion implantation into a monocrystal). The amount of impurity ions remaining in the polysilicon layer 58 varies. The amount of impurity ions remaining in the polysilicon layer 58 is referred to as a dose herein. Because of the variation of the dose of impurity ions, variation of the parasitic resistance Rpn tends to increase.
In the case of forming the n-type region 64 by implantation of a higher dose of n-type impurity ions than the dose of p-type impurity ions to previously form the p-type region 65, the n-type impurity ions compensate the p-type impurity ions and turn the same into n-type to form the n-type region 64. The formation of the n-type region 64 therefore depends on both of the doses of p-type impurity ions and n-type impurity ions. Accordingly, the variation of sheet resistance Rsn of the n-type region 64 is larger than the variation of sheet resistance Rsp of the p-type region 65. In the n-type region 64 formed by mutual compensation of the impurity ions as described above, carriers are scattered considerably, and the sheet resistance Rsn of the n-type region 64 is therefore higher than the sheet resistance Rsp of the p-type region. These sheet resistances Rsn and Rsp constitute the aforementioned parasitic resistance Rpn, which is expressed by the following equation:Rpn=Rsn×(Ln/W)+Rsp×(Lp/W)
Herein, Ln is the length of the current path in the n-type region 64. Lp is the length of the current path in the p-type region 65. W is width of the n-type region 64 and the p-type region 65. Moreover, Ln=Lp, and Ln+Lp=Lo.
Patent Literature (PTL) 1 describes a temperature sensing diode made of polysilicon in which the capacitances formed between p- and n-type regions implementing the temperature sensing diode and the substrate with the insulating layer provided just under the p- and n-type regions are set substantially equal to each other. This can prevent malfunction due to external noise.
PTL 2 discloses that the temperature sensing diode has a three-layer structure including p+ layer/p layer/n+ layer arranged in a planar direction.
PTL 3 discloses a structure of a temperature sensing diode in which the p- and n-type diffusion layers of the temperature sensing diode penetrate polysilicon vertically (the structure in which the diffusion layers reach the back surface of polysilicon).
PTL 4 discloses a temperature sensing diode using the change in avalanche voltage with temperature. In the temperature sensing diode, at least one of the p- and n-type regions is formed by introducing impurity ions with a dose of 5×1014/cm2 or less in order to obtain avalanche voltage that can steeply increase.